1. Field of the Invention
The present invention relate generally to a magnetic disk apparatus, read gate optimization method and program for demodulating read data from medium readout signals by setting a read gate, and more particularly, to a magnetic disk apparatus, read gate optimization method and program for optimizing start and end timings of the read gate by executing test read.
2. Description of the Related Art
Traditionally, in a magnetic disk apparatus, read execution time is determined by generating read gate signals based on sector pulses as a reference obtained from readout signals of servo information recorded on a medium, and typically, the read execution time is constant at any location on the medium. This is because the format of the magnetic disk apparatus itself is always constant in any data area on the medium. As a specific example, in the case of a format in which a sync byte portion (SB portion) is four (4) bytes, a data portion (Data) is 512 bytes, and ECC portion is 30 bytes,Read Time=PLO+SB+Data+ECC+a=(PLO+546+a) bytes  (1)It is noted that PLO is a necessary pull-in time for AGC and PLL of a read channel, and a is a time depending on a pipeline (usually, negative). The time to start reading is controlled on the upper side, for example within a hard disk controller (HDC). In the hard disk controller, a sector pulse, which is a reference clock for controlling the write and read timings, is generated by a window (servo gate) indicating the servo information, and a read start time, at which a read gate signal rises, is determined based on the sector pulse as a reference. The read start time is a value determined in order to read data written on the medium at an optimum position, and determining factors can include for example the above-described necessary pull-in time for AGC and PLL of a read channel, a delay time of a head IC at the time of writing, an encoder delay time of the read channel and a gap distance between a write head and a read head. On the other hand, a read end time, at which a read gate signal falls, will be after the read time of Equation (1) from the read start time. It is typical as techniques used in the conventional magnetic disk apparatuses to us the sector pulse as a reference for the read start time, start reading from the predetermined timing and terminate the reading after the elapse of a given time therefrom. Also, recently, a zone bit recording method is often used in which recording is performed by dividing radial direction into several zones and changing a transfer rate. Again, in this case, the inside of the magnetic disk apparatus is configured depending on a predetermined sector format at any location on the medium, and therefore, the read time is constant as long as the sector format is constant (See, e.g., Japanese Patent Application Laid-open Pub. No. Heill-31358).
However, in these conventional magnetic disk apparatuses, there are problems as follows.
(1) Problem likely to occur when the read start time is a specified value.
If a gap between a write head and a read head equipped on a head is larger, the data to be written on the medium will be written temporally later. Therefore, if reading started from the specified read start time, it will be impossible to read out from PLO position which is actually supposed to be read, and reading will be started earlier from the preceding position where PLO is not written. In this case, since foundation data before writing has been typically erased in terms of DC, DC erased portion is read when the reading Is started, therefore automatic gain control (AGC) can malfunction in demodulation IC. Such gap distance between a write head and a read head depends on the property of the heads which read and write data and varies according to manufacturers of the heads and differences between wafers.
(2) Problem likely to occur when the read end time is a specified value.
If the read end time is a specified value, as a possible phenomenon, when data to be written on the medium has been written temporally earlier, a data in PAD portion after ECC can not be read out. In this case, data which is different from the phase of written data have been written in the PAD portion, therefore miscorrection by a decoder will be caused in a read channel. In this way, miscorrection of ECC portion can be raised by the data. As a factor making data to be written on the medium earlier, there is a case that properties of heads and preamplifiers are changed by environmental temperature and others.
(3) Problem likely to occur according to improvement of format efficiency (insufficient PDA). As another factor, there is a case that the data to be read has not been written till the end, because of insufficiency of PDA portion. In recent apparatus, pipeline processing is operated to improve performance, wherein the read end time is terminated at the point of time when the necessary data has been read. The pipeline processing is to execute process for reducing wasteful time in last half of the reading as much as possible, and as an advantage, this leads to an increase of efficiency and an improvement of performance. But, in other words, Insufficiency of margin for ensuring the reading of the last half data is caused by this. Of course, if the last half of data is written in PAD portion with sufficient length, this will not be happen, but recently, the PAD portion tends to be reduced for improving the format efficiency as much as possible, so the PAD portion is becoming to be not always present sufficiently. Because of such insufficiency of the margin for the last half of data, last half of PAD portion in which data is not written is read out, and a data error, which is miscorrection of ECC in this case, is caused by this.